1. Field of the Invention
The present invention relates to infrared radiation detectors and, more particularly, to small bandgap semiconductor infrared detectors and methods of fabrication.
2. Description of the Related Art
Detection of infrared radiation emitted by warm bodies provides an important method for night vision (perception without visible light). Infrared detectors are of various types and include narrow bandgap semiconductors structured as photodiodes or photocapacitors. Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as the photosensitive narrow bandgap semiconductor. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.83 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows (3-5 .mu.m and 8-12 .mu.m) of greatest interest for infrared detectors.
An infrared imager incorporating an array of detectors with each detector a MIS photocapacitor in Hg.sub.1-x Cd.sub.x Te is disclosed in U.S. Pat. No. 4,684,812 (Tew and Lewis); FIGS. 1a-b are cross sectional elevation and plan views of a single detector. The detector operates as follows: a negative voltage (typically -2 volts relative to the Hg.sub.1-x Cd.sub.x Te substrate) is applied to the gate to form a depletion region in the n-type Hg.sub.1-x Cd.sub.x Te; the gate is then electrically isolated; incident infrared photons penetrate the gate and create electron-hole pairs in the Hg.sub.1-x Cd.sub.x Te; the photo-generated minority carrier holes accumulate in the Hg.sub.1-x Cd.sub.x Te at the interface with the gate insulator and form an inversion layer which reduces the size of the depletion region and lowers the absolute value of the gate potential; the gate potential is sensed which reveals the incident infrared photon flux; the inversion layer and depletion region are collapsed by applying a positive voltage to the gate; and then the cycle is repeated. The detector is typically operated at liquid nitrogen temperatures to limit the thermal creation of electron-hole pairs in the Hg.sub.1-x Cd.sub.x Te; a bandgap of 0.1 eV is only about 4 kT at room temperature.
An increase in the magnitude of the gate voltage implies a larger depletion region and better performance. However, too large a gate voltage leads to breakdown in the Hg.sub.1-x Cd.sub.x Te: electron-hole pairs are generated without any incident infrared photons which results in spurious detection results. The breakdown electric field is on the order of 1 Volt/.mu.m for narrow bandgap Hg.sub.1-x Cd.sub.x Te.
The probability that an incident infrared photon will create an electron-hole pair in the Hg.sub.1-x Cd.sub.x Te depends upon the wavelength (energy) of the photon and the bandgap of the Hg.sub.1-x Cd.sub.x Te. If the photon energy is less than the bandgap, then the probability of creating an electron-hole pair is essentially zero; if the photon energy is equal to or somewhat greater than the bandgap, then the probability is large and the photons have a penetration depth (1/e not absorbed) on the order of 1 to 10 .mu.m; and if the photon energy is much larger than the bandgap (such as three times the bandgap), then the probability of creating more than one electron-hole per incident photon becomes substantial and the penetration depth decreases to about 0.1 to 1 .mu.m. Blackbody radiation of a 300.degree. K. body has a spectral peak at about 10 .mu.m and drops rapidly for shorter wavelengths, so the flux of 8-12 .mu.m wavelength photons incident on a detector is typically much greater than the flux of 3-5 .mu.m wavelength photons. This means that if the detector is made of Hg.sub.0.8 Cd.sub.0.2 Te, then both 8-12 .mu.m and 3-5 .mu.m wavelength photons will be detected, but the 8-12 .mu.m wavelength photons will greatly outnumber the 3-5 .mu.m wavelength photons; whereas if the detector is made of Hg.sub.0.83 Cd.sub.0.27 Te, then only the 3-5 .mu.m wavelength photons will be detected. Essentially, only a limited range of wavelengths (a single "color") is detected, and to detect two (or more) infrared colors requires two (or more) MIS photocapacitors of differing bandgap Hg.sub.1-x Cd.sub.x Te alloys in the same detector.
Thus, the known MIS photocapacitor detectors have the problems of low breakdown electric field associated with the small bandgaps of Hg.sub.1-x Cd.sub.x Te alloys and single color sensitivity.